Secondary electron background

Finally, the width of the total UPS spectrum may be used to obtain a reasonable estimate of the work function of the solid sample under consideration49. The energy of the intensity cut-off of the secondary electron background in the UPS spectrum is... 

Electron induced Auger Electron Spectroscopy (EAES) makes use of high-energy electrons to remove core electrons via impact-ionization. However, in many instances the utility of EAES is limited by problems associated with the large secondary electron background and the lack of surface specificity inherent in the EAES excitation process [2, 8]. 

PAES (positron annihilation Auger electron spectroscopy) is another technique for producing true AES peak shapes that are free of the secondary electron background. PAES is identical to conventional Auger electron spectroscopy except that the sample excitation is done with a low energy beam of positrons rather than a higher energy beam of electrons. 

Fig. A.5 Post detection data treatment steps applied for MIE/UP spectra of adsorbed molecules, exemplaiily shown for for 0.125 TCE/SA on Mo(112). Raw spectrum (1), WF determination with differentiated spectrum and tangent (2). Shift from BE (a) to IP (b). Secondary electron background correction by subtracting a fitted polynomial function (3), background corrected spectrum (4), fitted spectrum and sum curve (5) gas phase spectmm on the bottom. Roman numerals on topof the bottom spectra assign the peaks of the corresponding gas phase EES spectra [23]. Reprinted (adapted) with permission from [24]—Copyright (2012) Elsevier...

Figure 1. X-ray excited Auger spectrum of polyethylene. Top trace raw data with secondary electron background subtracted (Ref. 3,5). Bottom trace Auger spectrum corrected for electron energy loss functions to the solid (Ref. 10). The corrected spectrum has a width of V 50 eV, compared to the 40 eV predicted by independent particle theory (see text).

Figure Bl.25.6. Energy spectrum of electrons coming off a surface irradiated with a primary electron beam. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmons) and to all kinds of inelastic process (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are more visible in a derivative spectrum.

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

Figure 7. Secondary electron micrograph and carbon (KLL) map of untreated catalyst A. photomicrograph obtained at 8kV, O.lnA primary beam B. corresponding carbon (KLL) Auger map obtained in (peak-background)/ background mode, at 8kv, lOnA primary beam.

In radiation chemistry, the track effect is synonymous with LET variation of product yield. Usually, the product measured is a new molecule or a quasi-stable radical, but it can also be an electron that has escaped recombination or a photon emitted in a luminescent process. Here LET implies, by convention, the initial LET, although the actual LET varies along the particle track also, the secondary electrons frequently represent regions of heterogeneous LET against the background of the main particle. 

The line width of the X-ray source is on the order of 1 eV for A1 or Mg Ka sources but can be reduced to better than about 0.3 eV with the use of a monochromator. A monochromator contains a quartz crystal which is positioned at the correct Bragg angle for A1 Ka radiation. The monochromator narrows this line significantly and focuses it onto the sample. It also cuts out all unwanted X-ray satellites and background radiation. An important advantage of using a monochromator is that heat and secondary electrons generated by the X-ray source cannot reach the sample. 

In electron excited spectra, Auger electrons are seen as small peaks on an intense background of secondary electrons originating from the primary beam. 

Figure 8 shows the attenuation length of electrons in solids as a function of their kinetic energy. The few theoretical calculations available cire in good agreement with these empirical data Only unscattered electrons convey useful information, while scattered electrons contribute to a structureless background (secondary electrons). From Fig. 8, it is clear that photoelectron spectroscopy probes at most a few tens of Angstroms. 

The most recent calculations, however, of the photoemission final state multiplet intensity for the 5 f initial state show also an intensity distribution different from the measured one. This may be partially corrected by accounting for the spectrometer transmission and the varying energy resolution of 0.12, 0.17, 0.17 and 1,3 eV for 21.2, 40.8, 48.4, and 1253.6 eV excitation. However, the UPS spectra are additionally distorted by a much stronger contribution of secondary electrons and the 5 f emission is superimposed upon the (6d7s) conduction electron density of states, background intensity of which was not considered in the calculated spectrum In the calculations, furthermore, in order to account for the excitation of electron-hole pairs, and in order to simulate instrumental resolution, the multiplet lines were broadened by a convolution with Doniach-Sunjic line shapes (for the first effect) and Gaussian profiles (for the second effect). The same parameters as in the case of the calculations for lanthanide metals were used for the asymmetry and the halfwidths ... 

Photomultipliers are vacuum tube photocells with a sealed-in set of dynodes. Each successive dynode is kept at a potential difference of 100V o that photoelectrons emitted from the cathode surface are accelerated M each step. The secondary electrons ejected from the last dynode are Collected by the anode and are multiplied so that a 10° — 107 — fold arnpli-t tfion of electron flux is achieved. This allows simple devices such as l croammeters to measure weak light intensities. Background thermal mission can be minimised by cooling the photomultiplier. The schematic... 

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